1. Field of the Invention
The present invention relates to a liquid ejection head, and an image forming apparatus comprising the liquid ejection head, more particularly to a liquid ejection head which prevents the evaporation of volatile solvent contained in ink.
2. Description of the Related Art
As an image forming apparatus, an inkjet recording apparatus or inkjet printer is known, which includes a liquid ejection head or inkjet printer head having an arrangement of a plurality of liquid ejection nozzles and which records an image on a recording medium by ejecting liquid or ink from the nozzles toward the recording medium while causing the inkjet head and the recording medium to move relatively to each other.
The inkjet head of the inkjet printer has pressure generating units, each including, for example, a pressure chamber to which ink is supplied from an ink tank through an ink supply channel, a piezoelectric element which is driven by an electrical signal in accordance with image data, a diaphragm which serves as a portion of the pressure chamber and deforms in accordance with the driving of the piezoelectric element, and a nozzle which is connected to the pressure chamber and from which the ink inside the pressure chamber is ejected in the form of a droplet due to the volume of the pressure chamber being reduced by the deformation of the diaphragm. In the inkjet printer, one image is formed on a recording medium by combining dots formed by the ink droplets ejected from the nozzles of the pressure generating units.
The ink comprises a dye or pigment, and a solvent, and the like, and when left for a long period of time in a dry atmosphere, the solvent evaporates, the viscosity of the ink increases, and the ink becomes highly viscous. In other words, if the ink is left in an unused state for a long period of time in the nozzles of the inkjet printer, then the solvent component contained in the ink evaporates from the surface of the ink in the nozzles, and the ink in the vicinity of the nozzles becomes highly viscous. If the ink becomes highly viscous in this way, then during normal driving by piezoelectric elements, the ink cannot be ejected from the nozzles and nozzle blockages occur. If a nozzle blockage of this kind occurs, it is not possible to record the pixels that are to be recorded by that nozzle, onto the recording medium, and therefore a white stripe, or the like, where nothing is recorded, occurs in the corresponding portion of the recording medium after printing, thus giving rise to a printing defect.
There is a method in which if a nozzle blockage has occurred, then the nozzle blockage is eliminated by using a suction pump or a pressure pump, or the like; however, if the ink has reached a very high viscosity, then it is difficult to eliminate blockages by this means, and consequently, the inkjet head ceases to function.
Therefore, as a method of preventing the evaporation of the solvent in the ink in the vicinity of the nozzles of the inkjet head, a method has been proposed in which humidified air is supplied to and expelled from the vicinity of the nozzles of the inkjet head, thereby keeping the vicinity of the nozzles in a humidified atmosphere (see, e.g., Japanese Patent Application Publication No. 2000-79696).
Moreover, various methods for recovering ink mist have been proposed, which provide technologies which are similar but do not prevent evaporation of the ink solvent (see, e.g., Japanese Patent Application Publication Nos. 2004-330446 and 2004-330615).
However, in Japanese Patent Application Publication No. 2000-79696, as well as supplying humidified air to the vicinity of the nozzles, the humidified air thus supplied is also expelled to the exterior of the apparatus, in order to prevent condensation. Therefore, if the apparatus is driven for a long period of time, then it is necessary to provide a large source of moisture, which is problematic in that it is impractical and increases the size of the apparatus, and furthermore, if the ink solvent, in other words, the moisture source, is an organic solvent such as alcohol, then the organic solvent is expelled to the exterior of the apparatus, which means that a disagreeable smell is generated whenever the apparatus is driven and the health of people in the vicinity may be damaged.
Therefore, Japanese Patent Application Publication No. 2000-79696 discloses that the whole of the head section is covered with a covering. However, when performing printing, the recording medium, such as paper, passes through the interior of the covering, and therefore the recording medium suffers deformation or alteration due to absorbing moisture from the humidified air. Furthermore, there is also a problem in that this also causes bleeding of the ink on the recording medium.
Furthermore, Japanese Patent Application Publication Nos. 2004-330446 and 2004-330615 both have recovery mechanisms only, and they are not directed toward the evaporation of the solvent from the ink.
As described above, methods have been proposed for supplying and expelling humidified air to and from the peripheral region of an inkjet printer head, but they involve many problems and are not practicable.